Wheels on the Teledyne -- need the right diameter seat post

Mavic rims with Dura Ace hubs (matches the Dura Ace rear deraileur)

The seat post is 26.8 mm (not the standard 27.2 mm).  I only have a 27.2 mm post, so I need to buy a smaller diameter post.  There's a crack on one side of one of the brazed on clamp tubes.  I'm hoping it won't break when tightened...